Military aircraft have used on-board inert gas generating systems (OBIGGS) for some years to protect against fuel tank explosions due to undesired phenomena, such as penetration from small arms fire. Military aircraft are not the only aircraft that would benefit from OBIGGS. For example, investigations into the cause of recent air disasters have concluded that unknown sources may be responsible for fuel tank ignition and explosion. Subsequently, OBIGGS has been evaluated as a way to protect commercial aircraft against such fuel tank explosions started by unknown ignition sources.
Ullage gas composition (i.e., the air/fuel mixture above the fuel in the fuel tank) consists of many constituents, including air components (oxygen, nitrogen, water vapor, carbon dioxide) and hydrocarbon compounds (fuel vapors). OBIGGS protects against fuel tank explosions by replacing the potentially explosive air/fuel mixture above the fuel in the tanks (the ullage) with an inert gas (usually nitrogen). The nitrogen is generated by separating oxygen from local, ambient air and pumping the inert product into the fuel tanks.
For OBIGGS, a means of verifying ullage oxygen concentration is necessary to verify proper operation. However, it is difficult to reliably measure oxygen concentration in the ullage without compromising safety. This is due in part to the fact that many conventional oxygen sensors operate at elevated temperatures, which could ignite hydrocarbon fumes in the fuel tank.
One approach used to monitor ullage oxygen concentration is to monitor the oxygen concentration at the output of the OBIGGS air separation module (ASM), where nitrogen enriched air (NEA) flows into the ullage at elevated pressure. The gas composition at the ASM output is nearly free of water and fuel vapors and is much easier to measure. U.S. Pat. No. 6,491,739 to Chrome et al. discloses a system that uses a conventional oxygen sensor to monitor ullage oxygen concentration at the ASM output. Oxygen concentration at the ASM output is a direct measure of OBIGGS operation and an indirect indication of the oxygen concentration in the ullage.